Improved nitric acid method crude phosphoric acid production process and denitration concentration system

Abstract

The invention relates to the field of phosphoric acid production, and particularly discloses an improved nitric acid method crude phosphoric acid production process and a denitration concentration system. The production process comprises the following steps: decomposing phosphorite with dilute nitric acid, freezing and crystallizing the obtained acidolysis solution, filtering, and adding concentrated sulfuric acid to obtain a mixed acid solution containing phosphoric acid and nitric acid; and introducing the mixed acid liquid into a first-effect evaporator and a second-effect evaporator for double-effect evaporation, and distilling in a rectifying tower to obtain a crude phosphoric acid product. According to the application, mixed acid is subjected to denitration concentration by adopting a downstream double-effect evaporation technology, by virtue of a pressure-temperature double-gradient design and in combination with a rectifying tower, staged denitration is realized by utilizing the pressure dependence of the volatility of nitric acid, and primary denitration and energy supply are realized by adopting a high-pressure and high-temperature evaporation section as a first-effect evaporation section; the second-effect evaporation is a vacuum low-temperature evaporation section, deep denitration and acid enrichment are achieved, and the concentration of nitric acid enriched in the low-temperature section reaches 55 wt%. The concentration of phosphoric acid in the finally obtained product is more than or equal to 75wt%, and the residual nitric acid is less than or equal to 0.5

Description

Technical Field

[0001] The present application relates to the field of phosphoric acid production, and more specifically, to an improved nitric acid process for crude phosphoric acid production and a denitrification and concentration system. Background Art

[0002] In the field of wet-process phosphoric acid production technology, with the continuous development of industry and the growing demand for phosphoric acid, the advancement of wet-process phosphoric acid production technology is particularly important. Currently, there are two common technical means in wet-process phosphoric acid production.

[0003] One method is to use sulfuric acid to decompose phosphate rock to produce a mixed slurry of phosphoric acid and calcium sulfate. By controlling the phosphoric acid concentration, temperature and free sulfuric acid concentration in the mixed slurry, three different calcium sulfate crystals are generated, namely calcium sulfate dihydrate, calcium sulfate hemihydrate, and anhydrous calcium sulfate. The calcium sulfate crystals (phosphogypsum) are then separated by filtration. The filtrate is crude phosphoric acid, which is then refined, impurity-removed, and evaporated for concentration to obtain qualified industrial phosphoric acid.

[0004] Another method is to use nitric acid to decompose phosphate rock to generate a mixed slurry of phosphoric acid, nitric acid and calcium nitrate. The acid-insoluble matter in the mixed slurry is first filtered to remove the acid-insoluble matter, and then the filtered mixed slurry is cooled to make most of the calcium nitrate generate calcium nitrate tetrahydrate crystals. The calcium nitrate crystals are separated by filtration, and concentrated sulfuric acid is added to the filtrate to convert the remaining calcium nitrate into calcium sulfate crystals and filtered to obtain a mixed acid solution containing phosphoric acid and nitric acid. For this mixed acid solution, common treatment methods include extraction process and single evaporation method. When using the extraction process, a specific extractant is required, and the extraction process requires multiple steps, such as extraction, back extraction, etc. The operation is relatively cumbersome, and the selection and use cost of the extractant is high. The single evaporation method is simply evaporation under a single condition, which makes it difficult to fully utilize the differences in boiling points and the ability to bind water molecules between phosphoric acid and nitric acid, resulting in incomplete separation of nitric acid and poor concentration of the liquid.

[0005] However, these existing technical methods have obvious drawbacks. The sulfuric acid method produces a large amount of phosphogypsum. Due to the difficulties in the comprehensive utilization of phosphogypsum, most of it can only be discharged as industrial solid waste, which puts great pressure on environmental protection. Although the nitric acid method can significantly reduce the production of phosphogypsum, the extraction process for treating mixed acid requires the use of specific extractants and multiple extractions, which is relatively cumbersome. At the same time, the selection and use costs of the extractants are high. When treating mixed acid by single evaporation, the nitric acid separation is not thorough, and the concentration effect of the liquid is poor. Summary of the Invention

[0006] In order to solve the problem, the present application provides an improved nitric acid process for crude phosphoric acid production and a denitrification concentration system.

[0007] This application adopts the following technical solutions:

[0008] In a first aspect, the present application provides an improved nitric acid process for producing crude phosphoric acid, comprising:

[0009] (1) decomposing phosphate rock with dilute nitric acid having a concentration of ≥65 wt%, and filtering to obtain an acid solution;

[0010] (2) freezing and crystallizing the acid solution under stirring, and crystallizing calcium nitrate in the form of calcium nitrate tetrahydrate, and filtering;

[0011] (3) adding concentrated sulfuric acid to the filtered liquid to convert the remaining calcium nitrate in the liquid into calcium sulfate dihydrate, and filtering to obtain a mixed acid solution containing phosphoric acid and nitric acid;

[0012] (4) passing the mixed acid solution into a first-effect evaporator for primary evaporation to obtain primary steam and a primary denitrification liquid; wherein the pressure of the first-effect evaporator is 90-98 kPa and the temperature is 105-112° C., and the primary steam contains 18-22 wt % of nitric acid;

[0013] (5) passing the primary steam into a distillation tower, controlling the tower top temperature at 95-99° C., and obtaining dilute nitric acid with a concentration of ≥55% (wt) at the tower bottom;

[0014] (6) The primary denitrification liquid is passed into a second-effect evaporator for secondary evaporation to obtain secondary steam and crude phosphoric acid; wherein the pressure of the second-effect evaporator is 6-8 kPa and the temperature is 75-80° C. The secondary steam contains 53-57 wt % of nitric acid, and after condensation, dilute nitric acid with a concentration of 53-57 wt % is obtained; the crude phosphoric acid contains ≥ 75 wt % of phosphoric acid and ≤ 0.5 wt % of nitric acid.

[0015] Furthermore, in the above step (4), the evaporation heat source of the first-effect evaporator is saturated steam of 0.3-0.5 MPa; in step (5), the steam after the primary steam is passed into the distillation tower for purification is used as the heat source of the second-effect evaporator; and the dilute nitric acid recovered in steps (5) and (6) is returned to step (1) for acid hydrolysis of phosphate rock.

[0016] Furthermore, the mixed acid solution contains 30-32 wt% phosphoric acid and 24-26 wt% nitric acid.

[0017] Furthermore, before the mixed acid solution is passed into the first-effect evaporator, the mixed acid solution is pretreated, including:

[0018] The mixed acid solution obtained in step (3) is heated to 40-50° C., sodium fluorosilicate seed crystals are added, and the mixture is reacted in a step flow reactor for 8-12 minutes, and the silicon slag precipitate is removed by filter pressing;

[0019] 0.04-0.06 wt% of EDTA-2Na is added to the filtered mixed acid solution, mixed and then passed into a first-effect evaporator.

[0020] Preferably, the sodium fluorosilicate seed crystals are pre-soaked in a 0.8-1.3% HF solution overnight before being added to the mixed solution; the added amount of the sodium fluorosilicate seed crystals is 1.5-2.5 g / L.

[0021] Furthermore, the method further comprises adding 0.4-0.6% formic acid into the second-effect evaporator in step (6).

[0022] Furthermore, in the above step (2), the process of freezing and crystallizing the acid hydrolyzed solution includes:

[0023] 0.05-0.1 wt% of sodium hexametaphosphate and 6-8 wt% of a low eutectic solvent are added to the acid hydrolysis solution at 20-25° C., and after ultrasonic dispersion, the temperature is lowered to 5-8° C. for pre-cooling to form seed crystals, and then the temperature is lowered to 0-2° C. for crystal growth, and then filtered.

[0024] Preferably, the deep eutectic solvent is obtained by mixing choline chloride and ethylene glycol in a mass ratio of 1:1.5-2.5.

[0025] In a second aspect, the present application further provides a denitrification and concentration system for the production of crude phosphoric acid by the nitric acid process, which is used to implement steps (4) to (6) in the above production process, and comprises a first-effect evaporator, a distillation tower, a second-effect evaporator, and a dilute nitric acid tank;

[0026] The mixed acid solution of step (3) is injected into the first-effect evaporator through the mixed acid feed pipeline and heated by the heating steam pipeline; the top gas outlet of the first-effect evaporator is connected to the feed inlet of the distillation tower, and the bottom discharge port of the first-effect evaporator is connected to the feed inlet of the second-effect evaporator;

[0027] The top outlet of the second-effect evaporator is connected to the condenser for purifying secondary steam, and the recovered nitric acid is fed into the dilute nitric acid tank; the bottom outlet of the second-effect evaporator is connected to the crude phosphoric acid outlet pipeline;

[0028] The top gas outlet of the distillation tower is connected to the heating steam pipeline of the second-effect evaporator, the bottom discharge port of the distillation tower is connected to the dilute nitric acid tank, and the dilute nitric acid tank is connected to the dilute nitric acid delivery pipeline.

[0029] Furthermore, the purified secondary steam passes through a steam ejector and returns to the first-effect evaporator to preheat the mixed acid liquid.

[0030] In summary, this application has the following beneficial effects:

[0031] 1. The improved nitric acid process for crude phosphoric acid production provided herein utilizes a co-current double-effect evaporation technique to denitrate and concentrate mixed acid. Through a pressure-temperature dual-gradient design and in combination with a distillation tower, denitration is achieved in stages, utilizing the pressure dependence of nitric acid volatility. The first-effect evaporation stage is a high-pressure, high-temperature evaporation section for initial nitrate removal and energy supply; the second-effect evaporation stage is a vacuum, low-temperature evaporation section for deep nitrate removal and acid enrichment. The low-temperature section enriches nitric acid to a concentration of 55 wt% (capable of direct reuse for acid-hydrolyzed phosphate rock). The final product has a phosphoric acid concentration of ≥75 wt% and a nitric acid residue of ≤0.5 wt%. Furthermore, the low temperature of the second-effect evaporation effectively inhibits phosphoric acid polymerization or decomposition, improving the quality of the final phosphoric acid product.

[0032] 2. Compared with the existing single evaporation process, the evaporation temperature of this application is low, the total amount of steam required is small, and the recovered nitric acid concentration is high; and the purified steam (105-112°C) after the first-effect evaporation can be directly used for the second-effect evaporation, and the steam is reused, which reduces the overall energy consumption by more than 45%.

[0033] 3. In a preferred technical solution, a small amount of formic acid is added to the second-effect evaporator as a nitric acid volatilization promoter, which can form a low-boiling-point azeotrope with nitric acid (the boiling point is reduced to about 53° C.). Continuing to use a temperature of 75-80° C. for the second-effect evaporation can increase the volatility of nitric acid, so that the residual amount of nitric acid in the final product phosphoric acid is reduced to ≤0.1%.

[0034] 4. In the preferred technical solution, the mixed acid solution is pretreated and sodium fluorosilicate seed crystals are added to remove the influence of colloidal silica on distillation; EDTA-2Na is added to the mixed acid solution as a phosphoric acid stabilizer to block the metal ion-catalyzed phosphoric acid polymerization, thereby improving the stability of phosphoric acid during the single-effect evaporation process.

[0035] 5. In the preferred technical solution, by adding sodium hexametaphosphate and a low eutectic solvent to the acid hydrolysis solution, the volume of the calcium nitrate crystals is increased, the viscosity of the liquid is reduced, and the crystallization temperature is increased, so that the refrigeration energy consumption of the calcium nitrate crystallization is greatly reduced, and the filtration efficiency is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a flow chart of the production process of crude phosphoric acid using the improved nitric acid process provided by the present application;

[0037] Figure 2 It is a system structure diagram for realizing the destocking and concentration step in the production process of this application.

[0038] Reference numerals:

[0039] Mixed acid feed pipeline 1; heating steam pipeline 2; first-effect evaporator 3; distillation tower 4; process water tank 5; second-effect evaporator 6; condenser 7; vacuum pipeline 8; crude phosphoric acid outlet pipeline 9; dilute nitric acid tank 10; dilute nitric acid delivery pipeline 11; dilute nitric acid pump 12; process water pump 13; process water delivery pipeline 14. DETAILED DESCRIPTION

[0040] The embodiments of the present invention will be described in detail below with reference to the examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Specific conditions not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0041] The technical solution of the present invention is:

[0042] The present invention provides an improved nitric acid process for producing crude phosphoric acid, which comprises the following steps:

[0043] (1) decomposing phosphate rock with dilute nitric acid having a concentration of ≥65 wt%, and filtering to obtain an acid solution;

[0044] The acid hydrolysis reaction is carried out under normal pressure with stirring, the reaction time is 1-3 hours, and the reaction temperature is controlled at 60-70° C. After the acid hydrolysis reaction is completed, the acid-insoluble matter in the feed liquid is separated by filtration.

[0045] The phosphate rock powder used in this application contains 31wt% phosphorus pentoxide and 47wt% calcium oxide as its main components. During the decomposition of the phosphate rock, the concentration of fresh dilute nitric acid used is ≥65wt%, which helps to increase the crystallization rate of calcium nitrate and reduce the emission of phosphogypsum.

[0046] (2) freezing and crystallizing the acid solution under stirring, so that calcium nitrate crystallizes in the form of calcium nitrate tetrahydrate and is filtered;

[0047] The acid solution is stirred and cooled by indirect heat exchange, causing calcium nitrate to crystallize as calcium nitrate tetrahydrate. The crystallization endpoint temperature is controlled between -10°C and -12°C, ensuring a calcium nitrate crystallization rate of 85% or higher. The calcium nitrate tetrahydrate crystals are then separated from the solution by filtration.

[0048] During the freeze crystallization of the acid solution, calcium nitrate crystallizes and precipitates in the form of calcium nitrate tetrahydrate. During this process, the calcium nitrate crystals are small in size (less than 50 μm) and easily form a network structure when the calcium ion concentration is saturated at low temperatures, resulting in low viscosity of the liquid and difficulty in effective filtration. At the same time, during the freeze crystallization process, the temperature needs to be lowered to -10 to -15°C and maintained for a certain period of time to allow the calcium nitrate tetrahydrate to precipitate, which consumes a lot of refrigeration energy and is not conducive to industrial production.

[0049] In order to simultaneously solve the problems of high viscosity and difficulty in filtering of the feed liquid, as well as low freezing crystallization temperature and high energy consumption, in a preferred embodiment of the present application, the process of freezing crystallizing the acid hydrolyzed liquid includes:

[0050] 0.05-0.1 wt% of sodium hexametaphosphate and 6-8 wt% of a low eutectic solvent are added to the acid hydrolysis solution at 20-25° C., and after ultrasonic dispersion, the solution is cooled to 5-8° C. for pre-cooling to form seed crystals, and then cooled to 0-2° C. for crystal growth, followed by filtration.

[0051] In this optimization scheme, the added sodium hexametaphosphate can chelate Ca in the feed solution. 2+ It reduces intercrystalline bridging and destroys the gel network structure, thereby reducing the viscosity of the liquid and improving the filtration efficiency. Preferably, the amount of sodium hexametaphosphate added is 0.07-0.08 wt%.

[0052] In this optimized solution, the addition of a deep eutectic solvent helps lower the eutectic point of the system, raising the crystallization temperature from -10-15°C to 0-2°C, thereby reducing the refrigeration load. Preferably, the deep eutectic solvent is a mixture of choline chloride and ethylene glycol in a mass ratio of 1:1.5-2.5. This composition exhibits good compatibility with the phosphorus system and can elevate the crystallization temperature.

[0053] In the above optimization scheme, pre-cooling at 5-8°C can generate seed crystals >50μm, and then slowly cooling to 0-2°C and keeping warm, the crystals grow to 100-150μm, the crystal volume becomes larger, and the viscosity of the liquid is low, making it easy to filter.

[0054] (3) adding concentrated sulfuric acid to the filtered liquid to convert the remaining calcium nitrate in the liquid into calcium sulfate dihydrate, and filtering to obtain a mixed acid solution containing phosphoric acid and nitric acid;

[0055] In the calcium sulfate crystallization step, concentrated sulfuric acid is added to the feed solution. The sulfuric acid reacts with the remaining calcium nitrate in the feed solution to produce calcium sulfate and nitric acid. The reaction is stirred for 8-12 minutes, and the calcium sulfate precipitates from the feed solution as calcium sulfate dihydrate crystals. Furthermore, the mixed acid solution contains 30-32 wt% phosphoric acid and 24-26 wt% nitric acid.

[0056] In order to separate nitric acid and phosphoric acid in a mixed acid solution, the present application abandons the existing solvent extraction method or single evaporation method and instead adopts a co-current double-effect evaporation technology for denitration and concentration. Through a pressure-temperature dual gradient design and combined with a distillation tower, denitration is achieved in stages, while the feed liquid is concentrated, and the phosphoric acid concentration in the final product is made ≥75wt% and the nitric acid residue is ≤0.5wt%. The evaporation temperature is low, and the polymerization or decomposition of phosphoric acid during the high-temperature evaporation process is effectively suppressed.

[0057] The steps of denitrification and concentration are as follows (4)-(6), including a three-stage system of first-effect evaporation, distillation, and second-effect evaporation. Specifically:

[0058] (4) passing the mixed acid solution into a first-effect evaporator for primary evaporation to obtain primary steam and a primary denitrification liquid; wherein the pressure of the first-effect evaporator is 90-98 kPa and the temperature is 105-112° C., and the primary steam contains 18-22 wt % of nitric acid;

[0059] The azeotropic point of nitric acid and water at normal pressure is 120°C (containing approximately 68% nitric acid). In order to avoid the impact of excessively high evaporation temperature on phosphoric acid, the present application adopts 90-98 kPa in the first-effect evaporation, so that the first-effect evaporation temperature can achieve efficient denitrification at 105-112°C. Nitric acid has a low volatility at high pressure, and the first-effect steam contains approximately 20 wt% nitric acid, thereby achieving preliminary denitrification.

[0060] Preferably, the evaporation heat source of the first-effect evaporator is saturated steam at 0.3-0.5 MPa;

[0061] (5) passing the primary steam into a distillation tower, controlling the tower top temperature at 95-99° C., and obtaining dilute nitric acid with a concentration of ≥55% (wt) at the tower bottom;

[0062] The primary steam from the first-effect evaporation (containing approximately 20wt% nitric acid) is distilled in a 95-99°C distillation tower. The resulting dilute nitric acid, with a concentration of ≥55wt%, can be directly reused in the acid hydrolysis step, avoiding the reduced acid hydrolysis efficiency associated with the traditional process of low-concentration nitric acid reuse. The top temperature of the distillation tower is controlled at 95-99°C, below the atmospheric azeotropic point, ensuring nitric acid enrichment while preventing azeotrope formation.

[0063] (6) passing the primary denitrification liquid into a second-effect evaporator for secondary evaporation to obtain secondary steam and crude phosphoric acid; wherein the pressure of the second-effect evaporator is 6-8 kPa and the temperature is 75-80° C., the secondary steam contains 53-57 wt % of nitric acid, and after condensation, dilute nitric acid with a concentration of 53-57 wt % is obtained;

[0064] Second-effect evaporation breaks the azeotropic equilibrium under low pressure. At 6-8 kPa (near vacuum), the azeotropic point of nitric acid and water drops to approximately 70°C. Using an evaporation temperature of 75-80°C significantly increases nitric acid volatility (the acid content in the secondary steam reaches 55%). The low evaporation temperature also prevents phosphoric acid from polymerizing or decomposing at high temperatures. Second-effect evaporation achieves deep nitric acid removal and acid enrichment.

[0065] Preferably, the steam (105-112°C) purified from the first effect by the distillation tower is used as the heat source for the second effect evaporator. The steam from the first effect directly drives the second effect evaporation, resulting in a high steam reuse rate. Furthermore, the low temperature of the second effect evaporation also effectively reduces cooling water usage.

[0066] In the above process, the residual amount of nitric acid in the final product phosphoric acid is ≤0.5%. In order to further improve the removal efficiency of nitric acid, in a preferred technical solution, 0.4-0.6% of formic acid is added to the second-effect evaporator in step (6). Formic acid, as a nitric acid volatilization promoter, can form a low-boiling-point azeotrope with nitric acid (the boiling point is reduced to about 53° C.). Continuing to use an evaporation temperature of 75-80° C. can increase the volatilization of nitric acid, so that the residual amount of nitric acid in the final product phosphoric acid is reduced to ≤0.1%.

[0067] In the aforementioned process, the second-effect evaporation process uses a low temperature (75-80°C) in a near-vacuum environment to prevent phosphoric acid decomposition or polymerization. However, in the first-effect evaporation process, due to the evaporation temperature (105-112°C), local overheating of the phosphoric acid during evaporation can easily produce pyrophosphoric acid (H4P2O7) as a byproduct, affecting product activity. Furthermore, the acid solution from the acid-dissolved phosphate rock contains a small amount of fluorosilicic acid (H2SiF6), which hydrolyzes at the high temperature of the distillation tower to form SiO2 colloids, which can deposit within the tower and create scaling risks.

[0068] In order to solve the above problems, in the preferred technical solution of the present application, before the mixed acid solution is passed into the first-effect evaporator, the mixed acid solution is further pretreated, including:

[0069] A. Heat the mixed acid solution to 40-50°C, add sodium fluorosilicate seed crystals, react in a step flow reactor for 8-12 minutes, and filter to remove silicon slag precipitation;

[0070] Before the acid hydrolysis solution enters the evaporation system, sodium fluorosilicate seed crystals (1.5-2.5 g / L) are added to induce the formation of Na2SiF6 precipitates, which are filtered out to avoid the subsequent formation of colloidal silica that affects distillation; preferably, before being added to the mixed solution, the sodium fluorosilicate seed crystals are pre-soaked in 0.8-1.3% HF solution overnight to form a porous surface and improve the induction efficiency.

[0071] B. Add 0.04-0.06 wt% of EDTA-2Na to the filtered mixed acid solution, mix well and pass it into a first-effect evaporator.

[0072] In the above steps, EDTA-2Na is added as a phosphate stabilizer, which can chelate Fe in the feed solution. 3+ / Al 3+ (forming a stable complex), blocking the metal ion-catalyzed phosphoric acid polymerization, thereby improving the stability of phosphoric acid.

[0073] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0074] Example 1

[0075] This embodiment provides an improved nitric acid process for producing crude phosphoric acid, and its process flow is as follows: Figure 1 Shown, including:

[0076] (1) Acid hydrolysis step:

[0077] Phosphate rock is decomposed using nitric acid. The concentration of fresh dilute nitric acid used is ≥65wt%. The phosphate rock powder used has a particle size of 80-100 mesh. The main components of the phosphate rock powder are 31wt% phosphorus pentoxide and 47wt% calcium oxide. The phosphate rock powder is metered and fed into the acid hydrolysis tank. Fresh dilute nitric acid and dilute nitric acid recovered in a subsequent step are metered and fed into the acid hydrolysis tank, controlling the excess nitric acid to 10%.

[0078] The acidolysis tank operates at a slightly negative pressure and is equipped with an agitator. Phosphate rock powder and dilute nitric acid are continuously fed, and the acidolysis solution is continuously discharged. The reaction proceeds under stirring for a total reaction time of ≥2 hours, with the reaction temperature controlled at 60-70°C. The slightly negative pressure in the acidolysis tank is maintained by an exhaust blower. The exhaust gas from the acidolysis reaction contains a small amount of nitric acid mist and NOx, which is sent to a scrubbing system for purification and discharge to meet emission standards.

[0079] The acid hydrolysis solution is a mixture of phosphoric acid, nitric acid, calcium nitrate, and water. After the acid hydrolysis reaction is completed, the acid-insoluble matter in the liquid is separated by filtration using a filter press.

[0080] (2) Calcium nitrate crystallization step:

[0081] The crystallizer is a normal pressure tank equipped with an agitator and internal cooling coils. The liquid from the previous step is fed into the crystallizer, where it is cooled by indirect heat exchange with a coolant. Crystallization occurs under stirring conditions, and calcium nitrate crystallizes as calcium nitrate tetrahydrate. The crystallization endpoint temperature is controlled at -10°C, ensuring a calcium nitrate crystallization rate of ≥85%.

[0082] After the calcium nitrate crystallization process is completed, the calcium nitrate tetrahydrate crystals in the liquid are separated by filtration; the filtration equipment uses a vacuum filter.

[0083] (3) Calcium sulfate crystallization step:

[0084] The crystallizer is a normal pressure tank equipped with an agitator. The feed solution from the previous step is fed into the crystallizer, and concentrated sulfuric acid is added to the feed solution. The sulfuric acid reacts with the remaining calcium nitrate in the feed solution to produce calcium sulfate and nitric acid. The reaction is stirred for 10 minutes, and the calcium sulfate precipitates from the feed solution as calcium sulfate dihydrate crystals.

[0085] After the calcium sulfate crystallization process is completed, the calcium sulfate dihydrate in the feed liquid is separated by filtration to obtain a mixed acid containing phosphoric acid and nitric acid (containing 31.6wt% phosphoric acid and 25.1wt% nitric acid); the filtration equipment uses a vacuum filter.

[0086] (4) Out-of-stock concentration step:

[0087] The system used in this step is Figure 2 As shown, it specifically includes a first-effect evaporator 3, a distillation tower 4, a process water tank 5, a second-effect evaporator 6, a condenser 7 and a dilute nitric acid tank 10;

[0088] The mixed acid liquid is injected into the first-effect evaporator 3 through the mixed acid feed pipeline 1 and heated by the heating steam pipeline 2; the top gas outlet of the first-effect evaporator 3 is connected to the feed port of the distillation tower 4, and the bottom discharge port of the first-effect evaporator 3 is connected to the feed port of the second-effect evaporator 6;

[0089] The top outlet of the second-effect evaporator 6 is connected to the condenser 7, and the recovered nitric acid is introduced into the dilute nitric acid tank 10; the purified secondary steam is discharged through the vacuum pipeline 8 and can be returned to the hot mixed acid liquid of the first-effect evaporator 3; the bottom outlet of the second-effect evaporator 6 is connected to the crude phosphoric acid lead pipeline 9;

[0090] The top outlet of distillation tower 4 is connected to heating steam line 2 of second-effect evaporator 6. The bottom outlet of distillation tower 4 is connected to dilute nitric acid tank 10, which is connected to dilute nitric acid delivery line 11. Dilute nitric acid pump 12 pumps the dilute nitric acid back into the acid hydrolysis tank for phosphate rock acid hydrolysis. During the distillation process, the process water reflux rate at the top of the distillation tower is controlled by adjusting the process water pump 13 and valve body of process water tank 5, thereby keeping the top temperature of distillation tower 4 within a preset range. The process water is then transported via process water delivery line 14.

[0091] The destocking and concentration step specifically includes:

[0092] Evaporation uses co-current double-effect evaporation technology to save steam. The mixed acid solution obtained in the above steps is fed into the tube side of the first-effect evaporator for evaporation, producing primary steam and a primary denitrified liquid. The denitrified liquid then flows by gravity into the tube side of the second-effect evaporator due to the pressure difference and continues to evaporate, producing secondary steam and evaporated liquid. The evaporated liquid is drawn out from the tube side of the second-effect evaporator to obtain the final product, crude phosphoric acid. The structural type of both the first-effect evaporator and the second-effect evaporator is a central circulation tube evaporator. The heating steam is 0.40MPa saturated steam, which is fed into the shell side of the first-effect evaporator and indirectly exchanges heat with the liquid in the tube side to heat the liquid.

[0093] The operating pressure of the first-effect evaporator tube side is 94 kPa and the operating temperature is 110°C. The primary steam from the first-effect evaporator contains 20wt% nitric acid and enters the distillation tower. The nitric acid in the primary steam is separated by distillation. The top temperature of the distillation tower is controlled at 97°C by adjusting the amount of process water reflux at the top of the tower. Dilute nitric acid with a concentration of ≥55wt% is obtained at the bottom of the tower. The purified primary steam is passed to the shell side of the second-effect evaporator as a heat source.

[0094] The operating pressure of the second-effect evaporator tube side is 8 kPa and the operating temperature is 77°C. The secondary steam from the second-effect evaporator contains 55% nitric acid by weight. After condensation, it produces dilute nitric acid with a concentration of 55% by weight. The recovered dilute nitric acid is returned to the acid hydrolysis step to continue decomposing the phosphate rock. The secondary steam, after recovering the nitric acid, passes through a steam ejector and returns to the first-effect evaporator to preheat the mixed acid solution; alternatively, it can be passed to the acid hydrolysis tank to provide heat in the acid hydrolysis step.

[0095] The evaporated liquid coming out of the second-effect evaporator is crude phosphoric acid, which contains phosphoric acid ≥75% (wt), nitric acid ≤0.5% (wt), and a small amount of impurities. It can be used to produce phosphate fertilizer products such as monoammonium phosphate and diammonium phosphate, and can also be refined and impurity-removed to produce industrial phosphoric acid.

[0096] Example 2

[0097] The difference between this embodiment and embodiment 1 is that in the denitrification and concentration step, the first-effect evaporation operation pressure is 94 kPa(A) and the operation temperature is 105°C; the second-effect evaporation operation pressure is 8 kPa(A) and the operation temperature is 75°C.

[0098] Example 3

[0099] The difference between this embodiment and embodiment 1 is that in the denitrification and concentration step, the first-effect evaporation operation pressure is 94 kPa(A) and the operation temperature is 112°C; the second-effect evaporation operation pressure is 8 kPa(A) and the operation temperature is 80°C.

[0100] Example 4

[0101] The difference between this embodiment and embodiment 1 is that 0.5 wt % of formic acid is added to the second-effect evaporator in the denitrification and concentration step, and the rest is the same as embodiment 1.

[0102] Example 5

[0103] The difference between this embodiment and embodiment 1 is that the mixed acid solution is pretreated before the denitrification and concentration step, specifically including:

[0104] (1) The mixed acid solution was heated to 45°C, sodium fluorosilicate seed crystals were added at a concentration of 2 g / L, and the mixture was reacted in a step flow reactor for 10 min, and then the precipitate was removed by filter press;

[0105] (2) Add 0.05 wt% of EDTA-2Na to the filtered mixed acid solution, mix well, and then pass it into a first-effect evaporator.

[0106] Example 6

[0107] The difference between this embodiment and embodiment 5 is that before the denitrification and concentration step, when the mixed acid solution is pretreated, the seed crystals added are sodium fluorosilicate seed crystals pre-soaked in 1.0% HF solution overnight, and the added amount is 2 g / L.

[0108] Example 7

[0109] The difference between this embodiment and embodiment 5 is that before the denitrification and concentration step, when the mixed acid solution is pretreated, no sodium fluorosilicate seed crystals are added, and only 0.05 wt % EDTA-2Na is added to the mixed acid solution, and the solution is mixed and then passed into a first-effect evaporator.

[0110] Example 8

[0111] The difference between this embodiment and embodiment 1 lies in the calcium nitrate crystallization step in step (2), specifically:

[0112] 0.07 wt% of sodium hexametaphosphate was added to the acid hydrolysis solution at 25°C and dispersed evenly; a low eutectic solvent formed by mixing choline chloride and ethylene glycol in a mass ratio of 1:2 was added in an amount of 7 wt%. After ultrasonic dispersion for 10 minutes, the temperature was lowered to 5°C and pre-cooled for 1 hour to form seed crystals with a size of about 50 μm. The temperature was then lowered to 0-2°C and maintained for 2 hours for crystal growth to obtain large-sized crystals of about 120-150 μm, which were then filtered.

[0113] Comparative Example

[0114] Comparative Example 1

[0115] The difference between this comparative example and Example 1 is that a single normal pressure evaporation process is adopted, and the evaporation temperature is 110°C.

[0116] Comparative Example 2

[0117] The difference between this comparative example and Example 1 is that a single-effect evaporation process is adopted, the evaporation pressure is 94 kPa, and the temperature is 110°C.

[0118] Comparative Example 3

[0119] The difference between this comparative example and Example 5 is that a double-effect evaporation process is adopted, and the pressure of the double-effect evaporation is 94 kPa and the temperature is 110°C.

[0120] Performance testing

[0121] By using the production processes provided in Examples 1-5 and Comparative Examples 1-2, equal amounts of phosphate rock were decomposed to prepare crude phosphoric acid. The phosphoric acid content and the residual nitric acid content in the crude phosphoric acid were measured. The crystal size of calcium nitrate tetrahydrate and the viscosity of the resulting feed solution in the calcium nitrate crystallization step, as well as the presence of scaling in the distillation column, were recorded. The results are shown in Table 1.

[0122] Table 1.

[0123]

[0124] As can be seen from Table 1:

[0125] In the crude phosphoric acid products obtained by the production processes provided in Examples 1-8 of the present application, the phosphoric acid content is ≥75wt% and the nitric acid content is ≤0.5wt%.

[0126] Combining Examples 1-3 with Comparative Examples 1-3, it can be seen that compared with single atmospheric pressure evaporation, single-effect evaporation (94 kPa, 110° C.) and isocratic double-effect evaporation (all 94 kPa, 110° C.), the gradient double-effect evaporation process of the present application is more thorough in removing nitric acid, and the nitric acid content in the obtained crude phosphoric acid is lower, and the phosphoric acid content is also improved.

[0127] Combining Examples 1-3 with Example 4, it can be seen that adding formic acid in the secondary evaporator can further promote the evaporation of nitric acid, and the nitric acid content in the obtained crude phosphoric acid is only 0.08 wt%.

[0128] Combining Examples 1-3 with Examples 5-7, it can be seen that pre-treating the mixed acid solution and adding EDTA-2Na before the denitrification and concentration step can effectively increase the phosphoric acid content in the crude phosphoric acid (by approximately 5 percentage points). At the same time, adding sodium fluorosilicate seeds to the mixed acid solution can effectively reduce scaling in the distillation column. Example 6 is the best, mainly because pre-immersing the sodium fluorosilicate seeds in HF solution helps increase the specific surface area of ​​the sodium fluorosilicate seeds, improves the efficiency of induced crystallization, and achieves good silicon removal effect.

[0129] Combining Examples 1-3 with Example 8, it can be seen that treating the acid solution during the calcium nitrate crystallization step can effectively increase crystal size and reduce the viscosity of the feed solution (by approximately 40%), thereby improving filtration efficiency. Furthermore, due to the presence of the deep eutectic solvent, the temperature of calcium nitrate crystallization is also raised to above 0°C.

[0130] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. An improved nitric acid process for producing crude phosphoric acid, characterized in that: It includes: (1) decomposing phosphate rock with dilute nitric acid having a concentration of ≥65 wt%, and filtering to obtain an acid solution; (2) freezing and crystallizing the acid solution under stirring, so that calcium nitrate crystallizes in the form of calcium nitrate tetrahydrate and is filtered; (3) adding concentrated sulfuric acid to the filtered liquid to convert the remaining calcium nitrate in the liquid into calcium sulfate dihydrate, and filtering to obtain a mixed acid solution containing phosphoric acid and nitric acid; (4) passing the mixed acid solution into a first-effect evaporator for primary evaporation to obtain primary steam and a primary denitrification liquid; wherein the pressure of the first-effect evaporator is 90-98 kPa and the temperature is 105-112° C., and the primary steam contains 18-22 wt % of nitric acid; (5) passing the primary steam into a distillation tower, controlling the tower top temperature at 95-99° C., and obtaining dilute nitric acid with a concentration of ≥55% (wt) at the tower bottom; (6) Passing the primary denitrification liquid into a second-effect evaporator for secondary evaporation to obtain secondary steam and crude phosphoric acid; wherein the pressure of the second-effect evaporator is 6-8 kPa and the temperature is 75-80° C., the secondary steam contains 53-57 wt% of nitric acid, and after condensation, dilute nitric acid with a concentration of 53-57 wt% is obtained; the crude phosphoric acid contains ≥75 wt% of phosphoric acid and ≤0.5 wt% of nitric acid.

2. The production process of crude phosphoric acid by the improved nitric acid process according to claim 1, characterized in that: In step (4), the evaporation heat source of the first-effect evaporator is saturated steam of 0.3-0.5 MPa; in step (5), the steam after the primary steam is passed into the distillation tower for purification is used as the heat source of the second-effect evaporator; the dilute nitric acid recovered in steps (5) and (6) is returned to step (1) for acid hydrolysis of phosphate rock.

3. The production process of crude phosphoric acid by the improved nitric acid process according to claim 1, characterized in that: The mixed acid solution contains 30-32 wt% of phosphoric acid and 24-26 wt% of nitric acid.

4. The process for producing crude phosphoric acid by the improved nitric acid process according to any one of claims 1 to 3, characterized in that: Before the mixed acid solution is passed into the first-effect evaporator, the mixed acid solution is pretreated, including: The mixed acid solution obtained in step (3) is heated to 40-50° C., sodium fluorosilicate seed crystals are added, and the mixture is reacted in a step flow reactor for 8-12 minutes, and the silicon slag precipitate is removed by filter pressing; 0.04-0.06 wt% of EDTA-2Na is added to the filtered mixed acid solution, mixed and then passed into a first-effect evaporator.

5. The production process of crude phosphoric acid by the improved nitric acid process according to claim 4, characterized in that: The sodium fluorosilicate seed crystals are soaked in 0.8-1.3% HF solution overnight before being added into the mixed solution; the addition amount of the sodium fluorosilicate seed crystals is 1.5-2.5 g / L.

6. The process for producing crude phosphoric acid using the improved nitric acid method according to any one of claims 1 to 3, characterized in that: The method further comprises adding 0.4-0.6% formic acid into the second-effect evaporator in step (6).

7. The process for producing crude phosphoric acid using the improved nitric acid method according to any one of claims 1 to 3, characterized in that: In step (2), the process of freezing and crystallizing the acid hydrolyzed solution comprises: 0.05-0.1 wt% of sodium hexametaphosphate and 6-8 wt% of a low eutectic solvent are added to the acid hydrolysis solution at 20-25° C., and after ultrasonic dispersion, the solution is cooled to 5-8° C. for pre-cooling to form seed crystals, and then cooled to 0-2° C. for crystal growth, followed by filtration.

8. The improved nitric acid process for producing crude phosphoric acid according to claim 7, characterized in that: The deep eutectic solvent is obtained by mixing choline chloride and ethylene glycol in a mass ratio of 1:1.5-2.

5.

9. A denitrification and concentration system for the production of crude phosphoric acid by the nitric acid process, for implementing steps (4) to (6) in the production process according to any one of claims 1 to 8, characterized in that: It includes a first-effect evaporator, a distillation tower, a second-effect evaporator, and a dilute nitric acid tank; The mixed acid solution of step (3) is injected into the first-effect evaporator through the mixed acid feed pipeline and heated by the heating steam pipeline; the top gas outlet of the first-effect evaporator is connected to the feed inlet of the distillation tower, and the bottom discharge port of the first-effect evaporator is connected to the feed inlet of the second-effect evaporator; The top gas outlet of the second-effect evaporator is connected to the condenser for purifying secondary steam, and the recovered nitric acid is introduced into the dilute nitric acid tank; the bottom discharge port of the second-effect evaporator is connected to the crude phosphoric acid lead pipeline; The top gas outlet of the distillation tower is connected to the heating steam pipeline of the second-effect evaporator, the bottom discharge port of the distillation tower is connected to the dilute nitric acid tank, and the dilute nitric acid tank is connected to the dilute nitric acid delivery pipeline.

10. The denitrification and concentration system for crude phosphoric acid production by the nitric acid process according to claim 9, characterized in that: The purified secondary steam passes through a steam ejector and returns to the first-effect evaporator to preheat the mixed acid solution.

Images